Since rubber products such as rubber belts and rubber tires are subjected to high tensile force when used, cords composed of twisted yarns made of glass fiber or aramid fiber are used as reinforcing member to be embedded into such rubber products. Such a cord is made of glass fiber or aramid fiber in the following manner. That is, a primary twist is imparted to filaments of the fiber such that the filaments are twisted into twisted yarns and a final twist is imparted to the plural twisted yarns such that the twisted yarns are further twisted together into a cord. The characteristics of the cord are controlled by changing the condition of the primary twist and the final twist and/or the combination therebetween. For example, by increasing the twisting rate of the primary twist and the final twist, the flexural fatigue resistance of the cord is improved. This is because of the following reason. That is, when the cord made of twisted yarns is bent, the bent portion is subjected to tensile force at the outside thereof and is subjected to bucking force at the inside thereof. Since the higher twisting rate facilitates the expansion and contraction of the twisted yarns, the aforementioned tensile force and bucking force are dispersed and thus received by the entire cord. On the other hand, by decreasing the twisting rate in the primary twist and the final twist, the dimensional stability of the cord is increased. The reason can be easily understood from the fact that the elongation of a reinforcing member with fiber which is not twisted at all is equal to the elongation of the fiber itself.
A cord, in which the direction of the final twist is the same as the direction of the primary twist, has excellent flexural fatigue resistance. This is because of the following reason. That is, as the cord made of the twisted yarns is twisted only in one direction wholly, the twisted yarns made by the primary twist are further twisted in the same direction by the final twist, thereby exhibiting the similar effect of the aforementioned increase in the twisting rate. As an example of such reinforcing members, Japanese Utility Model Publication No. S59-15780 discloses a reinforcing member made of glass fiber manufactured by imparting a primary twist to its filaments such that the filaments are twisted into twisted yarns and by imparting a final twist to the twisted yarns such that the twisted yarns are twisted in the same direction as the primary twist.
A cord, in which the direction of the final twist is opposite to the direction of the primary twist, has excellent dimensional stability. This is because of the following reason. That is, twisted yarns made by the primary twist are twisted in the opposite direction by the final twist, thereby exhibiting the similar effect of the aforementioned decrease in the twisting rate of the primary twist.
In view of the aforementioned concerns between the primary twist and the final twist, the improvement in the flexural fatigue resistance and the retention of the high dimensional stability of the cord are in relation contradicting each other. It seems quite difficult to achieve the both of them concurrently.